Photocatalysis is a promising method for energy-saving environmental purification. We have found that oxygen-deficient titanium dioxide samples prepared by radio-frequency plasma treatment have photocatalytic activity not only under illumination of ultraviolet light but also with visible light (400-600 nm). The new photocatalyst, which will soon be supplied by a collaborating company, is expected to work more efficiently under the sun as well as in the indoor environment.